The invention relates to coating paper webs with an aqueous coating mass or slip, also referred to as coating paint.
The coating paints comprise either one inorganic pigment or a mixture of different inorganic pigments which are bound in one organic binder vehicle or in a mixture of organic binder vehicles. It is known to apply these coating paints onto the paper webs with a so-called film press also as metering size press also known as metering size press. For this purpose the coating paint is applied to a rubber roller of the film press on one or both sides of the web, whereupon the paint is transferred by this roller or these rollers onto the paper web under pressure. When the paper web coated with the coating mass or paint, exits from a nip between the rollers of the film press, a splitting or separation of the coating mass film occurs due to coating disturbing forces generated in the nip exit area because at least some of the coating mass tends to stay on the coating roller rather than be transferred to the web being coated. Thus, a separation of the coating mass within itself occurs. The coating mass is more or less viscous and known rheologic problems occur due to said coating disturbing forces which lead to so-called flow lines or to the orange peel effects which reduce the quality of the coating.
Moreover, such film presses run with a speed of 800 to 2000 m/min., whereby small coating mass particles pass as free fog or spray fog into the space just downstream of the nip where the coating mass splits apart on its way from the wetted roller to contact the paper web due to said speed which increase the coating disturbing forces. Conventionally one must put up with this fog more or less and control it in practice by holding a black cardboard in the vicinity of the nip exit area of the paper web on its way to a drying station. Thus, efforts have been made to influence the adhesion forces between the coating paint and the roller as well as the cohesion forces within the coating paint in such a way that the fog at the nip exit and the so-called orange peel pattern are reduced or avoided. However, these efforts have not led to satisfactory results in practice.
The invention has for its object to improve the coating surface of produced paper webs and to minimize the generation of spray fog and the so-called orange peel effects.
The above objects have been achieved according to the invention by the combination of the following method steps. First, the charge status or charge polarity of the paper web and of the coating mass are ascertained. Second, the web is passed through the nip between two rollers forming part of a film press or metering size press that performs the coating at a web speed of at least 800 m per minute. Third, the paper web or the coating mass or both are so charged that each has a polarity opposite to the polarity of the other. Fourth, as the coating proceeds in the film press, the opposing charges or polarities generate attraction forces that oppose or counteract the above mentioned coating disturbing forces that are effective in the nip exit area downstream of the nip as viewed in the moving direction of the paper web. The attraction forces thus reduce or even eliminate the above mentioned spray fog and orange peel effects.
The present method is performed by an apparatus according to the invention which is characterized by the following features: a coating station including at least one coating film press for coating at least one surface of said paper web with said coating mass, said film press comprising two rollers forming a nip and nip exit area downstream of said nip as viewed in a moving direction of said paper web through said film press, a drive for moving said paper web through said nip and nip exit area in said moving direction at a web speed of at least 800 meters per minute, and an electric charging device positioned for ionizing at least one of said paper web and said coating mass with a charge polarity so that said paper web and said coating mass have opposing charge polarities at least in said nip exit area for acting against coating disturbing forces tending to disturb said coating mass on said paper web in said nip exit area to reduce spray fog and orange peel effects.
The invention has the advantage that with this method using an aqueous coating mass a more closed that is a smooth surface of the coating is achieved which in turn results in an improved printability of the paper. Furthermore, one achieves with the method according to the invention a printability that was possible heretofore only with thicker coatings. As a result, according to the invention printing products can be produced having a lower mass or weight per square. Since these film presses apply the aqueous coating mass, thinner coatings have been made possible, whereby the printed matter produced with such thinner coatings can be advantageously disposed in an environmentally friendly manner.
Furthermore, due to the suppression of the spray fog, less coating mass and fewer cleaning stops are required for the coating equipment, whereby the costs of the thus-coated papers can also be reduced.